680 NEW YOKE STATE MUSEUM 



escence is " saltpeter ", and when it occurs in burning tlie manu- 

 facturer at times erroneously ascribes it to water-smoking. 



The efSorescence is usually due to the presence of soluble salts, 

 specially sulfates, which are formed either in the clay or during 

 some stage of the manufacture. Any moisture present in the clay 

 or product dissolves these compounds and on evaporation carries 

 them to the surface of the ware. 



The subject has been discussed in some detail in the Brifh- 

 huilder, from which the following points are taken. -^ 



1 Pormation of efflorescence in the clay beds, etc. Most clays con- 

 tain mineral salts in greater or less quantities, which chemical 

 analysis has shown to be sulfates of lime and magnesia, less fre- 

 quently of iron and alkalis. The formation of these sulfates is 

 generally due to the decomposition of iron pyrite contained in the 

 clay, and it will be seen that the more thoroughly this material is 

 distributed throughout the clay the more easily it will be subjected 

 to complete decomposition, and the greater amount of soluble sul- 

 fates will be formed. All clays do not contain iron pyrites. In any 

 one clay bank the pyrites may be more abundant in so-me layers than 

 in others. It may be present in equal quantities in all layers, but 

 its decomposition may have proceeded tO' a greaiter extent in those 

 beds which are the most weathered. This fact has been brought out 

 by Dr Gerlach's observations. OnCi of these was that clay which, 

 had been allowed to lie for months in the open air left behind on 

 the ground where it had been large quantities of beaultiful gypsum 

 crystals; but the omission of the intermediate operation of allowing 

 the clay to weather after it has been dug will not necessarily pre- 

 vent the formation of these soluble sulfates, for the same decompo- 

 sition of the pyrites may occur if the green bricks are allowed to 

 stand a long time in the drying-room, in the presence of moisture. 

 The prevention therefore would see^m to^ be in the ordinary molding 

 of the clay and the drying and burning of the bricks as quickly as 

 possible. This oxidation and decomposition of iron pyrites is there- 



1 O. Gerlach. BricJciuilder. 1898. p. 59, et seq. 



